>I'm interested to know the best spices for bread.

Best spices is a relative term in cookery and baking....it wholly
depends upon your taste!

>I've been making heavy bread with whole-wheat, oats and nuts. Some recipes
>call for 5 spice and cinnamon with fruit. Anything else good to throw in
>there?

The use of spice combination requires care so that one particular spice
does not overpower the other but blend in harmony.
The rule in spice use in breadmaking is done ( figuratively?) with a
'heaping tablespoon of imagination' and a pinch of caution.!

>I've been making bread with onion/tomato/cheese, which has come out quite
>nice, but I'd like to give it a little more flavour, what spices do you
>suggest.

It depends upon your taste and you can play around with adding
coriander, Szechuan pepper and hot chillies...
To give you an idea about the actual usage of spices in breads:
A lot of the spices that can be used for cookery can also be used in
breadmaking, but the application is preferably done in harmony with
other ingredients. to contribute a unique flavor and taste.
Among the proven application of spices for breads are
=EF=82=A7 Garlic is roasted and can be added in walnut sourdough. Garlic c=
an
also be added in cheese- garlic bread
=EF=82=A7 candied Ginger can be mixed with chocolate chips and incorporated
to the dough
=EF=82=A7 Cardamom is used in Danish pastries
=EF=82=A7 Caraway is added to rye bread and even cumin as well where they a=
re
combined also as topping to some flat breads and even with sesame seed
topping on some ciabatta.
=EF=82=A7 Chopped jalapeno chilies and even habanero . and birds eye
chilies can be added in olive- walnut-olive sourdough
=EF=82=A7 Some ethnic Asian bread recipes uses curry powder in their breads
=EF=82=A7 Five spice powder and Szechwan pepper is also used in oriental ty=
pe
of bread buns
=EF=82=A7 The Japanese seven spice mix is also added in some oriental type
bread
=EF=82=A7 Nutmeg , allspice and mace can be used in Danish pastry as well
combined with citrus peel..These spices can be used to flavor the
donuts as well.
=EF=82=A7 Springerle spices in sweet dough.
=EF=82=A7 Sweated onions is used with pitted Calamata olives in focaccia
=EF=82=A7 Black pepper, coriander seed and cumin can also be added in
focaccia and ciabatta
=EF=82=A7 Mixed spice in fruited sweet buns
=EF=82=A7 Coriander is used is Borodinksy rye bread.
There are lots of other spice application in breadmaking that only
depends upon your imagination and if you start thinking creatively like
the Iron Chef< grin>
Roy